In recent years, sensitive materials permitting ample freedom in the design of functions, particularly electrophotographic sensitive materials provided with a laminate type sensitive layer permitting division of functions by the use of an electric charge generating material adapted to generate electric charge on exposure to light and an electric charge transferring material to perform transfer of the generated electric charge, have been proposed as electrophotographic sensitive materials. In the specification of Japanese patent application Disclosure SHO 57(1982)-144,558 and Japanese patent application Disclosure SHO 61(1986)-62,038, there are disclosed electrphotographic sensitive materials which have superposed on an electroconductive substrate an electric charge generating layer containing as an electric charge generating substance a square phosphoric acid derivative and an electric charge transfer layer containing such a diamine derivative as 4,4'-bis[N-phenyl-N-(3-methylphenyl)amino]diphenyl. Other electrophotographic sensitive materials which are equivalent to those just mentioned, excepting cyanine type compounds, azo type compounds, and such perylene type compounds as N,N'-bis(2,4,6-trimethylphenyl)perylene-3,4,6,10-tetracarboxydiimide, N,N'-diphenylperylene-3,4,9,10-tetracarboxydiimide, and N,N'-dimethylperylene-3,4,9,10-tetracarboxydiimide are used instead as electric charge generating substances, are disclosed in the specifications of Japanese patent application Disclosure SHO 57(1982)-144,556, Japanese patent application Disclosure SHO 57(1982)-144,557, Japanese patent application Disclosure SHO 60(1985)-207,418, Japanese patent application Disclosure SHO 61(1986)-275,848, and Japanese Patent Application Disclosure SHO 61(1986)-132,955.
The sensitive materials having such a laminate type sensitive layer as described above are allowed to divide the function of electric charge generation and the function of transfer of generated electric charge by the use of the aforementioned electric charge generating layer and electric charge transfer layer and, therefore, have an advantage that they generally possess high sensitivity and permit ample freedom in the selection of raw materials therefor. Since the electric charge transferring substances used therein are generally positive hole transferring substances, however, these sensitive materials are positively charged with difficulty. As regards the positively charging toner for developing an electrostatic latent image formed on the sensitive material, the freedom in the selection of raw materials for the toner is restricted because this toner is obtained with great difficulty. An attempt at resorting to the negative charging by means of corona discharge proves to be undesirable from the standpoint of safety and hygiene because the corona discharge entails generation of ozone. Further, the use of the corona discharge inevitably requires the image forming device such as a copying machine to be provided with a passage to be used for expulsion of the generated ozone and, consequently, necessitates an increase in size of the device. The electric charge generating layer has a small thickness on the order of 1 to 2 .mu.m and, therefore, must be formed with high accuracy. Since the sensitive material necessitates superposition of the electric charge generating layer and the electric charge transfer layer one on top of the other, this superposition inevitably increases the number of steps in the process for the production of the sensitive material, with the result that the operational efficiency will be impaired, the yield lowered, and the cost increased. Electrophotographic sensitive materials of the type provided with a single-layer type sensitive layer jointly containing an electric charge generating substance, an electric charge transferring substance, and a binding resin have been known to the art. The sensitive material of this type have an advantage that it can be charged positively and can prevent generation of ozone, enjoys ample freedom in the selection of raw materials for the toner, and permits the production thereof to be carried out with high efficiency. If, for furtherance of this advantage, the same electric charge generating substance and electric charge transferring substance as used in the laminate type sensitive layer mentioned above are used in the single-layer sensitive layer, the sensitive material using this single-layer type sensitive layer raises a problem that it exhibits no sufficient electrophotographic property. The single-layer type sensitive layer, unlike the laminate type sensitive layer, has both the electric charge generating substance and the electric charge transferring substance contained in a mixed state therein and does not easily permit division of the function of electric charge generation and the function of electric charge transfer. Thus, it suffers from restricted freedom in the selection of raw materials, low sensitivity, and high residual potential. Moreover, the electrophotographic properties of this sensitive material are heavily affected by the particular combination of an electric charge generating substance and an electric charge transferring substance. The aforementioned diamine derivative as an electric charge transferring substance exhibits meager dependence of drift mobility upon field intensity. The sensitive material furnished with the single-layer type sensitive layer containing the diamine derivative, therefore, is expected to exhibit small residual potential. When the single-layer type sensitive layer is produced by combining the diamine derivative with a varying perylene type compound as an electric charge generating substance, for example, the sensitive material furnished with this single-layer type sensitive layer raises a problem that it suffers from yet high residual potential and insufficient sensitivity and fails to exhibit sufficient electrophotograhic properties.
As regards the sensitive material using a single-layer type sensitive layer, the specification of Japanese patent application Disclosure SHO 56(1981)-143,438 discloses a sensitive material which is furnished with a single-layer type sensitive layer containing a polycyclic aromatic hydrocarbon as an electric charge generating substance and polyvinyl carbazole as an electric charge transferring substance.
In this sensitive material, however, since the positively charging property is not sufficient, the polyvinyl carbazole as the electric charge transferring substance exhibits low draft mobility. Since this sensitive material exhibits heavy dependency of the drift mobility upon the field intensity, it suffers from large residual potential and low sensitivity and exhibits no sufficient electrophotographic property.
For the purpose of offering an electrophotographic sensitive material which makes the most of the advantage of the single-layer type sensitive layer, excels in charging property and sensitivity, and enjoys low residual potential, the inventors formerly proposed a sensitive material furnished with a single-layer type sensitive layer produced by combining a perylene type compound like N,N'-bis(3,5-dimethylpheny)Perylene-3,4,9,10-tetracarboxydiimide with a hydrazone type compound like N-methyl-3-carbazolylaldenhyde-N,N-diphenyl hydrazone (Japanese patent application SHO 62(1987)-107,780).
The hydrazone type compound so used still suffers from heavy dependency of the draft mobility upon the field intensity, high residual potential, and insufficient sensitivity. While the hydrazone type compound has an advantage that it exhibits a lower degree of photoisomerization on exposure to light than 4-(N,N-diethylamino)benzaldehyde-N,N-diphenyl hydrazone, it raises a problem that it possess no sufficient photostability and suffers from gradual decrease of sensitivity and gradual increase of residual potential due to repeated use.